How to deal with your friend cheating?

My best friend cheats on her amazing husband all the time. She lives off of him he's much older then we are but I feel sick knowing she's cheating at first I assumed they must have an open relationship because she did it so often. I resently was out to dinner with them both and he was telling my husband and I how much in love with her he is and can't even think of being with anyone else then she said the same and my husband and I just looked at each other and went with it. I don't know how to put up with her cheating on him my husband told me just to stop talking to her. To cut her out of my life but it's hard cuz we been friends for so long. What should I do?